Do not destroy external linkage when deleting function body

The function deleteBody() converts the linkage to external and thus destroys
original linkage type value. Lack of correct linkage type causes wrong
relocations to be emitted later.
Calling dropAllReferences() instead of deleteBody() will fix the issue.

Differential Revision: http://reviews.llvm.org/D5415


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218302 91177308-0d34-0410-b5e6-96231b3b80d8